Fiber based passively mode-locked ps lasers are of considerable interest as cheap and robust master oscillators for high-power ps and fs master-oscillator power amplifier (MOPA) systems. In particular, oscillators mode-locked by semiconductor saturable absorber mirrors (SESAMs) have been found to provide easy self-starting and good environmental stability. Considerable effort has recently been put into the development of Yb-based fiber oscillators capable of delivering high-quality self-similar pulses with a large bandwidth in both the ps and fs range.
Apart from bandwidth and pulse quality, oscillators for practical use must also satisfy strict requirements on lifetime and stability. The latter is especially important if the output pulses are to be strongly amplified. With respect to lifetime, the SESAM is a particularly critical component. To stabilize the laser the SESAM must usually be operated in a regime of strong saturation, where the increase in reflectivity with pulse energy levels out. This in turn requires a high light intensity on the device, thus reducing the lifetime. During use, the performance of most saturable absorbers degrades resulting in an increase in the required gain in the cavity which is sufficient to achieve mode-locked operation. In many configurations of mode-locked fibre lasers this increase may be compensated by increasing the pumping of the gain medium. However, at some point mode locking is no longer possible, at which point the life time of the saturable absorber may be considered to have expired. In some cases the saturable absorber becomes insensitive to the incoming optical power. The saturable absorber may also expire instantly if the impingent optical power exceeds its damage threshold. Sometimes this occurs if the laser is allowed to Q-switch with a high gain in the gain medium of the laser.
The present inventors have found that the life time of mode locked fibre lasers utilizing a saturable absorber is often limited by the life time of the saturable absorber. In many cases life times are in the order of hundreds of hours which is insufficient for most commercial applications of the laser.